For some time, it has been desirable for electrical and electronic devices to communicate and share information, particularly in environments where such devices are operated within a single facility such as an industrial plant, office building or residence. To this end, a need has been discussed for electronic communications to occur between products such as data terminals, remote printers, personal computers, entertainment systems, refrigerators, washers, dryers lights, security and temperature control systems all of which may be operated within a single facility.
Various schemes have been proposed to accomplish such inter-element communication, including forms of radio transmission, light transmission and dedicated communication networks. It has also been suggested in the past that such inter-element communication can be efficiently accomplished utilizing in situ power distribution networks. For example in U.S. Pat. No. 4,697,166--Warnagiris et al., incorporated herein by reference, describes one such system for inter-element communication system, known as the ANI PLC (Power Line Carrier) system.
As recognized in U.S. Pat. No. 5,471,190--Zimmermann, incorporated herein by reference, in the late 1980's, there was a growing interest in communication networks for home automation systems and products to facilitate the interconnection of lights, appliances, sensors and control devices. One system, critically described in this patent was the so-called X-10 system. Eventually, an industry standard was adopted by the Electronics Industries Association which standard was called CEBus (Consumer Electronic Bus).
The CEBus protocol is described in U.S. Pat. No. 5,471,190 as being a low cost, low speed local areal network that uses a power line carrier to send control information over typical house electrical power wiring. Data is transmitted in packets utilizing spread spectrum techniques, such as those described in U.S. Pat. No. 5,090,024--Vander May et al., incorporated herein by reference, utilizing carrier frequencies between 100 kHz and 400 kHz.
One universal problem with all such inter-element communication schemes is noise present on the power lines which interferes with the communication signals. Such noise is generally attributed to motors and electronic switching devices attached to the power network. In addition to the patents already referenced, this problem is also highlighted in U.S. Pat. Nos. 4,815,106--Propp et al. and 5,452,291--Eisenhandler et al., both of which are incorporated herein by reference. Indeed, U.S. Pat. No. 4,815,106 specifically states that to overcome the noise problem and make the "smart home" a reality, less expensive equipment is needed to connect consumer devices to CEBus networks.
Early installations of CEBus power line devices demonstrated difficulties in communicating across phases of the typical 120/120/240 volt secondary service provided by distribution transformers. A generally adopted solution to this problem was power and communication coupling between line voltage and neutral. However, this solution did not prove sufficiently successful in cross phase communication. For example, in a residence environment, difficulties would occur when communications were attempted between a device connected to the 120 volt circuit for one floor and a device connected to the 120 volt circuit for another floor. Each of U.S. Pat. Nos. 4,815,106 and 5,471,190 disclose the line/neutral coupling technique.
Accordingly, a need still exists for apparatus and methods which will facilitate the accurate transmission of communication signals in power line networks.